The explosions triggered an intense fire that quickly engulfed the vessel. The submarine fire took about two hours to fully extinguish.

Speaking on condition of anonymity, an Indian official told the AP that divers have yet to recover bodies from the diesel-powered submarine.

"We hope for the best but we have to prepare for the worst," Admiral D.K. Joshi, the navy chief, told reporters earlier in Mumbai.

The explosion, which was captured on video, initiated in the submarine's torpedo compartment and gave way to a massive fireball that went hundreds of feet into the air and sent sailors on nearby vessels jumping into the water to avoid the flames, the AP reported.

"It's a loss to all of us. It's a tragedy," Defense Minister A.K. Antony told reporters in New Delhi before leaving for Mumbai to assess the situation.

The minister confirmed that some sailors were killed, but would not elaborate further, the AP reported.

Though it is unclear how much damage the explosion caused at the naval base, Joshi told reporters that the blast triggered secondary weaponry explosions in nearby areas.

The cause of the explosions is currently under investigation, according to Indian Naval spokesman Narendra Vispute.

The 16-year-old INS Sindhurakshak, which was Russian-made, suffered a less significant explosion in 2010. The initial explosion was reportedly caused by a faulty battery valve that leaked hydrogen.

The 2010 blast killed one sailor and injured two others, the AP reported.

Following the 2010 explosion, the submarine was sent to Russia for repairs where it was refitted and upgraded. It was returned to India in April of this year, and had been certified for use by the Indian navy, according to Joshi.

Zvyozdochka, the Russian ship repair company that upgraded the vessel, has already claimed the blasts were in no way related to its repair work, the AP notes.

The explosions come at a time when India is rapidly expanding its Navy, having in 2012 acquired a Russian Nerpa nuclear submarine on a 10-year lease at a cost of nearly $1 billion, activating the sub's atomic reactor this past Saturday. According to Indian officials, the nuclear submarine will be ready to deploy it in the next two years.